1. Field of the Invention
The present invention deals with the field of steam turbines.
2. Brief Description of the Related Art
Steam turbine rotors and inner casings, when the turbine is starting up, are subject to high thermal stresses, in particular in the region of the inlet, from the relatively hot steam flowing past them, and these stresses limit the service life of the components and the start-up time.
Therefore, various proposals have already been made in the past as to how the rotors and inner casings of steam turbines can be cooled in the critical areas without additional, external devices.
U.S. Pat. No. 4,551,063 has disclosed a medium-pressure steam turbine in which cooling steam is removed at the outlet of the high-pressure turbine prior to the reheating and is routed out of an annular space located outside the steam passage, via axial bores in the rotor, into the first two stages of the turbine, where it is fed into the steam passage from the blade roots. A solution of this type can only be employed for high-pressure turbines but not for medium-pressure turbines.
In the case of a combined high-pressure/medium-pressure steam turbine disclosed by U.S. Pat. No. 5,149,247, the stator is divided into an external stator and an internal stator, which are separated from one another by an intermediate space. For cooling purposes, cooling steam is removed from the final stage of the high-pressure part and introduced into the intermediate space. A similar solution is also disclosed in U.S. Pat. No. 6,341,937. Neither solution prevents the whole of the inner stator being exposed to the live steam.
Finally, in U.S. Pat. No. 6,010,302, the rotor is provided with a central bore through which cooling steam which has been removed at the outlet of the high-pressure stage is routed. In this solution, cooling of the inner casing is not provided and is indeed not possible.